The present invention relates to an optical communication system for executing an optical communication between vehicles such as satellites, space stations, space shuttles by use of a space propagation.
Generally, in an optical communication system, there is used an optical fiber type in which communication stations are connected to each other with an optical fiber cable, the optical fiber cable is used as a transmission line to execute an optical communication between the communication stations. In this type of the optical communication system, the capacity of communication can be largely increased as compared with the conventional RF communication.
In the field of the space development, the diversity of communication such as a satellite communication has been recently developed, and an increase in capacity of communication is required. In the field of the space development, there is an idea in which the optical communication system is constructed between the space vehicles to improve the communication capacity.
As such an optical communication system, there is considered and studied a system in which communication light is transmitted to a communication counterpart by use of a space propagation without forming an optical fiber cable. In such an optical communication system, light propagated in space is received and transmitted by an optical antenna to be guided to an optical signal processing system. In this case, the directivity of the optical antenna is controlled to a communication direction with highly accuracy.
For example, as shown in FIG. 5, a housing 1 is provided in a space vehicle 50 through gimbals 2 for coarsely tracking to be freely tracked. The housing 1 has an optical antenna 3. Also, the housing 1 has a tracking mirror 4 and a first beam splitter 5. The tracking mirror 4 is formed such that one input/output (I/O) optical path faces to the optical antenna 3. The first beam splitter 5, which constitutes a light receiving optical system, is formed on the other I/O optical path to face to the input line.
The gimbals 2 controls the movement of the housing 1 based on a coarse tracking command from a sensor (not shown). For example, the directivity of the optical antenna 3 is coarsely tracked to a communication counterpart station 52, which is constructed in the other space vehicle 51.
An image forming lens 6 is provided on one output line of the first beam splitter 5, and an optical receiving section 7 such as an APD (Avalanche Photo Diode) is provided on the back stage of the lens 6. A receiving signal processing section 8 is connected to the output terminal of the optical receiving section 7. The optical receiving section 7 photoelectrically converts input receiving light and outputs it to the receiving signal processing section 8.
A light angle detecting section 9 is formed on the other output line of the first beam splitter 5 through a beam splitter 10. A drive controlling section 11 is connected to an output terminal of the light angle detecting section 9. A receiving light from the first beam splitter 5 is guided through a second beam splinter 10, and the light angle of the receiving light is detected by the light angle detecting section 9. Then, light angle data is output to the drive control section 11. The drive control section 11 generates a tacking mirror drive signal based on input light angle data, and controls a tracking angle of the tracking mirror 41 thereby precisely tracking the communication counterpart station 52.
An image forming lens 13 is provided on an input line of the second beam splitter 10 through an aberration correction mirror 12. A light transmitting section 14 such as LD (Laser Diode) is provided on the back stage of the image forming lens 13. The light transmitting section 14 converts an electrical signal input through a transmitting signal processing section 15 to light so as to be supplied to the image forming lens 13.
The above transmitting light is guided to the optical antenna 3 through the aberration correcting mirror 12, the second beam splitter 10, the first beam splitter 5, and the tracking mirror 4 so as to be transmitted toward the communication counterpart station 52. In this case, the angle of the aberration correcting mirror 12 is controlled based on the space vehicles 50 and 51, and the aberration of the transmitting light is corrected.
The aberration data is calculated by a calculating section 17 based on light angle data detected by the light angle detecting section 9 and light data of transmitting light reflected by a corner cube reflector (CCR) 16 provided on the other output terminal of the second beam splitter 10.
Also, the housing 1 has a beacon optical system 18 facing to the optical antenna 3. The beacon optical system 18 is connected to a beacon beam emitting section 19. Beacon light is transmitted from the beacon beam emitting section 19 to an optical antenna 3.
In the above-explained optical communication system, the rate of communication data is high, and the using current of the light receiving section 7, serving as a heat source, and that of the light transmitting section 14 become large current. Due to this, measurements against heating must be taken to prevent the optical system from being influenced by heating. Moreover, since modulation frequencies of the signal processing sections 8 and 15 are increased, measurements against electromagnetic interference must be taken. Due to this, there occurs a problem in which the size of the device is increased, and its weight is increased.
Furthermore, it is difficult to position the light receiving section 7 and the light transmitting section 14 not to exert unfavorable influence on the optical system. Due to this, there occurs a problem in which a frequency characteristic is reduced.
The above-mentioned problems are the important subjects to be solved in the field of the space development, which demands the diversity of communication and the reduction in the size and weight of the device.